420 Coolant Expansion / Header Tank

Fitting the coolant expansion tank was one of those tasks I personally dreaded, but like a number of such task it went smoothly and its been the so called easier stuff which has been a pain.


What makes fitment of the coolant expansion more tricky on a 420 is that you can’t use the normal chassis mounting located at the centre of cruciform just in front of the engine. That space and mounting is used by the dry sump oil tank. What’s required is the fitment of an additional support bracket to the right hand side, the bracket needs to be mounted to the top of chassis tubing using rivets, rivets which need holes, and the holes require you to drill into the chassis tubing. This is the part that I was personally not looking forward to, as it happens I shouldn’t have worried.


Caution is the word here!


The bracket used has four holes, two are aligned on one tube while the other two are aligned on another, the threaded hole in the middle of the bracket faces downwards. When you know which way the bracket fits, the setup is pretty obvious. The tricky bit is aligning all four holes so that they are also in the top middle of the chassis tubing, on my S3 chassis this didn’t seem particularly difficult, although I believe it is more challenging on an SV. The manual does make it clear the first and forward most hole must be 135mm from the cruciform centre, this measurement seemed pretty good when used and all for four holes aligned perfectly.


I used some masking tape wrapped around the tubing which I carefully marked to denote the location of each hole. I then use a punch to mark an initial guide for the drill. Given the round nature of the tube I was concerned the drill might slide sidewards, to avoid this I started with a small 1mm pilot hole where the punch had marked and then stepped up to the final 4mm drill bit. Everything went to plan, and I manage four cleanly drilled holes in the centre of the tubing, with the front most be 135mm from the centre.


Next up was the straightforward task of inserting four rivets and attaching. I used my air rivet gun which made quick work of this step.

Lastly fix the smaller bracket to the tank and then attach to the main bracket now attached to the chassis. I didn’t photograph this step, although the additional bracket which holds the plastic tank is pretty straightforward.


The final result should look something like this.


You can also fit the replacement expansion bottle cap at this point.

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Meet Mark
Loves cars, light weight specials, jDM, Lotus, Caterham, Rallying and Trackdays